While an induction heating type of heating apparatus is generally known as a heating section of a hot plate, electric oven, or the like, investigations have in recent years been actively pursued into the use of an induction heating type of heating apparatus as a heating section in an image forming apparatus such as a copier, facsimile machine, or printer.
In a fixing apparatus that uses an induction heating type of heating section, magnetic flux generated by a magnetic flux generation section is made to permeate a heat-producing layer of a heat-producing element, and the heat-producing layer is made to produce heat by means of an eddy current generated by the permeation of this magnetic flux. Then an unfixed image formed on recording paper such as copy paper or an OHP sheet is directly or indirectly heat-fixed by heat of the heat-producing element heated by heat production of this heat-producing layer.
Specifically, for example, a heat-producing layer of electrically conductive material is formed on a heat-producing element comprising a fixing roller, fixing belt, or the like. Also, the heat-producing element and a pressure roller on either side of the recording paper feed path are positioned so as to be pressed together, forming a nip that grips and transports recording paper. Furthermore, an exciting coil is wound around a core of ferromagnetic material, forming a magnetic flux generation section, and the exciting coil is positioned opposite the heat-producing layer of the heat-producing element. Then an alternating current of predetermined frequency is applied to the exciting coil, and magnetic flux is generated around the exciting coil, forming a magnetic field, and the heat-producing layer of the heat-producing element is made to produce heat by means of an eddy current generated by the action of this magnetic field. In this state, recording paper is transported to the nip between the heat-producing element and pressure roller, and an unfixed image on the recording paper is fixed by heat of the heat-producing element heated by heat production of the heat-producing layer and pressure of the pressure roller.
An advantage of a fixing apparatus that uses this induction heating type of heating section, compared with a heat roller type of fixing apparatus that uses a halogen lamp as a heat source, is that heat production efficiency is higher and the time required for heating to a predetermined temperature can be shortened.
In an induction heating type of heating apparatus, as described above, when an alternating current flows in the exciting coil, a magnetic field is generated and magnetic flux penetrates the heat-producing layer. As a result, an eddy current I is generated in the heat-producing layer by electromagnetic induction, and heat RI2 proportional to resistance R of the heat-producing layer is generated.
Therefore, with this kind of heating apparatus, when the exciting coil is positioned opposite the heat-producing element, the larger the value of exciting coil resistance R, the higher is the heat production efficiency of the heat-producing layer.
This exciting coil resistance value R is known to be highest when the thickness of copper serving as the heat-producing layer is 5 (μm). Therefore, with this kind of induction heating type of heating apparatus, the heat production efficiency of the heat-producing layer is optimal when a heat-producing element is used in which the thickness of copper serving as the heat-producing layer is 5 (μm).
A known conventional fixing apparatus configured with attention paid to this kind of relationship between the thickness of the heat-producing layer and heat production efficiency is a fixing apparatus that uses an electrically conductive layer with a thickness of approximately 5 (μm) as the heat-producing layer (copper) of the heat-producing element (heating belt) (see Patent Document 1, for example). In the fixing apparatus described in Patent Document 1, a heating belt on which an electrically conductive layer is formed by vapor deposition of highly electrically conductive copper to an extremely small thickness of approximately 5 (μm) on a polyimide base material is used as the heat-producing element.
Patent Document 1: Unexamined Japanese Patent Publication No. 2004-145368